I don't know if Ford uses the same basic trip computer software on all of its vehicles or not, so this may not be valid for your truck. With that said, I've noticed on the Lincoln Town Cars at work (everything from 2004 to 2011 model years) that it seems to be based on three factors. First is the "Average fuel mileage" reading. Second is how low you get the tank before refuelling it. Third is where the "Miles to Empty" was beforehand. Again, at least with the Town Cars, the higher your fuel mileage reading is and the lower you get your fuel tank before stopping for fuel, the higher your "Miles to Empty" reading will be. Also, with both of those factors in place, it'll go up to an extent, but it won't go up too much compared to the previous reading...you need to drive the vehicle on the highway for a few tanks and get the tank to 1/4 or lower to get the "Miles to Empty" up significantly.
Hey, playing with the trip computer gives me something to do when there's no traffic and my client is sound asleep and I can't have the music on
Again, I don't know if the F-series trip computer has anything in common with the Town Cars' computers, although IIRC the trip computer in the Expedition is very similar to the Town Cars'...